A polycrystalline silicon thin-film transistor has been known in which a polycrystalline silicon film is formed as a semiconductor layer on the surface of an insulating substrate, and the semiconductor layer is used instead of a conventional semiconductor substrate. In such a polycrystalline silicon thin-film transistor, a silicon crystal grain boundary limits carrier mobility, and thus it is desirable to uniformly form polycrystalline silicon having a grain size as large as possible. However, it has been difficult for a so-called laser recrystallization technique, in which amorphous silicon as a starting material is heated and melted by irradiation with laser, then recrystallized when being cooled, to control the temperature of a silicon melted portion stimulating crystal growth. Therefore, it has been difficult to form crystals having a large grain size stably and uniformly.
With respect to the above, first, there is a technique disclosed in Japanese Patent Laying-Open No. 2000-12484 (patent document 1). In patent document 1, amorphous silicon is irradiated with an overlap of a first pulse laser beam having a wavelength in a visible range and a second pulse laser beam having a wavelength in an ultraviolet range generated by subjecting the first pulse laser beam to wavelength conversion into its second harmonic, and the second pulse laser beam is applied to be included in the first pulse laser beam in terms of time and space on the amorphous silicon to suppress dissipation of heat quantity from the silicon melted with the second pulse laser beam. Thereby, the temperature of a melted portion is controlled to form a polycrystalline silicon film having a relatively large grain size.
Second, there is a technique disclosed in Japanese Patent Laying-Open No. 2000-286195 (patent document 2). In patent document 2, an Nd:YAG2ω laser beam, which is visible light laser, is used. The laser beam is focused in a form of a fine line having a light intensity distribution in a substantially Gaussian shape in a width direction, and applied as a laser beam having an energy density gradient of a certain level or more on amorphous silicon. In this technique, a visible laser beam having a low absorption coefficient in amorphous silicon is employed to suppress a temperature gradient in a film thickness direction and to form a temperature gradient intentionally in the width direction, causing a one-dimensional lateral growth. With this structure, a polycrystalline silicon film having an array of crystals having a large grain size is obtained.
Patent document 1: Japanese Patent Laying-Open No. 2000-12484
Patent document 2: Japanese Patent Laying-Open No. 2000-286195